Recently, the processes for manufacturing high-frequency devices have been developed to make circuits more integrated by increasing the operation frequency of the circuits and continuously reduce the manufacturing cost of the high-frequency circuits accordingly. Further, the development of the processes allows for high-frequency circuits with a low power consumption characteristic by making it easy to lower power voltage.
FIG. 1 is a diagram illustrating the configuration of a linear amplifier according to the related art and FIG. 2 is an exemplary diagram illustrating gain characteristic of the linear amplifier illustrated in FIG. 1. Referring to FIG. 1, in a linear amplifier 100 of the related art, a common source transistor 110 and a common gate transistor 120 are connected in a cascode type. The gate of the common source transistor 110 is connected with an input node 105 and a capacitor 155 is connected between the gate and the input node 105, such that a DC component in an input can be blocked. Further, in the common gate transistor 120, a gate voltage is applied to the gate through a gate voltage node 125 and a reference voltage VDD is applied to the drain through a reference voltage node 135. In this case, an inductor 145 is connected between the reference voltage node 135 and the drain of the common gate transistor 120.
The reason of configuring the liner amplifier in a cascode type can be largely divided into two cases. One is for increasing the gain of the amplifier by configuring the linear amplifier in a cascode type. The other is for protecting the transistors in the linear amplifier. Since linear amplifiers usually generate large output power, the magnitude of the voltage at the output node increases and the drain-source voltage of the transistors in the linear amplifiers considerably drops, such that transistors may break due to overloads when it is over the yield voltage that the drain-source can support. This phenomenon can be attenuated in the linear amplifier of a cascode type, because the common source transistor and the common gate transistor, which are connected in series, distribute a voltage.
Referring to FIG. 2, P1dB power, a reference for determining linearity of a linear amplifier, means an input power point when a 1 dB gain reduces in comparison to the gain in the linear section. In the gain curve (a) for a linear amplifier of a cascode type, it can be seen that the gain reduces at input power of a predetermined level or more. This means that linearity is not maintained any more. Accordingly, it may be possible to increase the size of the common gate transistor of the linear amplifier illustrated in FIG. 1 in order to improve linearity of the linear amplifier. In the gain curve (b) of a linear amplifier with the size increased, however, there is a problem in that although the P1dB power increases, the gain characteristic decreases at the points with low input power.